In a generally known configuration, a touch panel is provided on a surface of a display device included in portable equipment such as smartphones or tablet computers. The touch panel is used as an input means for detecting a contact between a finger or a pointer and the touch panel. To detect position of the finger or the pointer on the touch panel, a method of detecting a change in electrostatic capacitance is mainly used, in which the change in electrostatic capacitance is caused by a contact between the touch panel and a finger or a pointer.
However, with a configuration of a display device provided with a touch panel, the thickness or weight of the entire display device increases. Hence, in the configuration of such a display device, the touch panel may be an undesirable component. In this respect, an organic film has been mainly used for the touch panel in order to reduce the weight of the display device provided with a touch panel. Even with such a touch panel, it is difficult to avoid increase in the thickness of the entire display device. Further, the display device provided with the above-described touch panel and high definition pixels has a disadvantage that necessary input (e.g., pen-input) to the touch panel is difficult to achieve.
Specifically, when a display device has high definition pixels such as of 300 ppi (pixel per inch) or 400 ppi or more, the pixel pitch ranges around 10 μm to 30 μm. Thus, when display devices are provided with the above-described touch panel and high definition pixels, most of the touch panels cannot tolerate a pen-pressure. Further, it is difficult to achieve required resolution sufficiently suitable for a high-definition display device. Accordingly, touch sensing technique of the touch panel is required to be more sophisticated.
In recent years, development of a so-called in-cell type touch sensing technique is underway. In the technique, touch sensing function is provided to the inside of the liquid crystal cell or to the display device without using a touch panel (hereinafter referred to as in-cell display device).
A well-known configuration of the above-described display device includes a color filter substrate where a plurality of colored layers are regularly arranged, and an array substrate where active elements such as TFTs (thin film transistors) are internally arranged.
A prototype in-cell display device includes an in-cell structure in which a pair of touch sensing electrodes are provided either on a color filter substrate or an array substrate, or provided on both of the color filter substrate and the array substrate. According to the structure, a touch sensing function for detecting an input position of a finger or a pointer or the like can be accomplished by detecting a change in electrostatic capacitance produced between the touch sensing electrodes.
Liquid crystal display devices used for mobile devices, such as smartphones or tablet computers generally employ a method of using a fringe electric field produced between pixel electrodes and a common electrode to drive horizontally-aligned liquid crystal cells in a direction relative to the horizontal electric field. The method is referred to as an FFS (fringe field switching) or IPS (in-plane switching) method, and has a feature of securing a wider viewing angle than in a method of driving liquid crystal cells in the vertical electric field direction. Each of the pixel electrodes included in the array substrate of the liquid crystal display device is in a comb-teeth pattern or configured of a plurality of stripe patterns having slits. The common electrode is provided under the pixel electrodes.
Display devices or touch panels disclosed in PTLs 1 to 5, for example, are known as conventional display devices or touch panels.